Water flow in the Amur-Heilong basin varies widely between seasons and years. At Komsomolsk City on the lower Amur average annual flow is 10,900 m3/sec. Maximum flow is 37,900 m3/sec and minimum recorded flow is just 345 m3/sec, less than 1% of the maximum. In Russia it is believed that large floods occur once every 11-13 years in the middle Amur. Summer monsoon rains occur across most of the basin and cause the floods that are common in most Amur-Heilong basin rivers. Floods are one of the most important natural processes and determine, in part, the diversity and productivity of the Amur-Heilong ecosystems. The shaping and dynamics of the vast floodplain wetlands, the major nutrient cycles, and the life-cycles of all aquatic flora and fauna depend primarily on the periodicity, volume, and other characteristics of floods.
Water levels in the upper and middle reaches of the basin vary over a range of 10-14 m during the year. In the lower Amur, the water level range is 6-7 m. On average there are 4-6 floods each year, increasing to 6-9 on small rivers. During floods the water surface of the lower and middle Amur-Heilong may expand to widths of 10-25 km. Waters often remain on the floodplain for extended periods.
Flooding is considered not only one of the most important ecological characteristics of the Songhua River basin; it is also the most important economic problem. Floods recur regularly on the Nen and Songhua plains and are the primary agent that shaped the plains and that led to the creation of its extensive wetlands. Several million hectares of the largest inland freshwater wetlands of China are located on the Song-Nen plain and on the Sanjiang plain

In June to August 1998, the most severe flooding on record in the Nen and Songhua River basins was caused by simultaneous monsoon rainfall in the headwaters of both rivers. Floods of this magnitude are estimated to recur on the average of only once in 150 years. The 1998 flood created lakes larger than 8,000 km2 in Jilin and Heilongjiang Provinces. More than 7.54 million people were relocated to higher ground, some of whom were still waiting 6 months later for waters to recede before they could return to their homes and villages. Water-logging lasted for two years on some parts of the flood plain.
A total of 154 people lost their lives during the 1998 floods. The 1998 floods affected most of the Song-Nen plain in eastern Inner Mongolia Autonomous Region (IMAR), western Heilongjiang Province, and northern Jilin Province. Damage costs were estimated at $1.8 billion, $3.6 billion, and $1.7 billion, respectively. The floods caused incomes of 1.8 million people in the three provinces to fall below the government poverty threshold. The floods disrupted social and economic activities of some 16.1 million people. There was extensive damage to houses, crops, livestock, fish farms, commercial premises, and infrastructure including roads, bridges, railways, power transmission, irrigation systems, water storage and reticulation, sewerage reticulation, drainage systems, and flood protection facilities. Floods damaged over 937,000 hectares of farmland, 2,600,000 hectares of grasslands, 89,000 hectares of fish ponds and 126 water reservoirs (Beach 1999).
The 1998 floods together with similar events in the Yangzi, Liao and other river basins pushed Chinese water management authorities to rethink the paradigm of "flood prevention" and to shift from engineering approaches toward adaptation to natural processes.

Climatic fluctuations often show cyclical patterns within short timeframes. The most obvious example of this is the cyclical pattern of water-abundant and water-deficit periods in Amur-Heilong River flow data at Khabarovsk. During the past 110 years, full cycles can be observed in the periods of 1924-1944, 1936-1955, and 1955-1979. Water-abundant periods occurred in 1896-1916, 1926-1943, 1955-1966, while water-deficit periods were 1917-1927, 1967-1980.

Figure: Volume of Amur-Heilong River flow at Khabarovsk hydrological station 1895-2001 (Novorotsky 2002)

A more complicated pattern of precipitation cycles has been observed in the area extending from eastern Mongolia to Lake Khanka. Small, 9-12 year cycles coincide with the intensity of solar radiation, while more pronounced 20-25 year cycles resulted in minimum precipitation in the beginning of the 20th Century, in the 1920s, mid-1950s, late 1970s, and the early 21st Century. While the tendency is uniform for the Amur-Heilong basin, the onset of some drought periods may be delayed for 2-3 years. Eastern monsoon regions enter drought periods later and return to wet periods earlier than western locations such as Chita, Mongolia, and Inner Mongolia. In western part of the basin where huge wetlands and lakes such as Dalai or Torey regularly dry up once in several decades the role of draught cycle in local ecosystem process is especially obvious and impressive.
Population dynamics and migration patterns of many species are closely linked to these cycles, as demonstrated for cranes, stork, bustard, and even musk deer.
It has been calculated that in China economic loss from draughts is much greater than from floods. Draught effects are prolonged over time and thus are less noticeable than floods, which are viewed as natural catastrophes. Both types of losses demonstrate that human economic development has failed to adapt to climate patterns peculiar to the region. Inefficient, high-levels of water consumption that are possible in wet periods cannot be sustained in times of draught without severely dropping water levels in wetlands and streams. Development of floodplains occurs during draught periods, buildings and infrastructure that are then destined to be damaged by next flood.